The present invention relates generally to drill bits, and more specifically relates to drill bits and methods for their construction which include an improved cutter configuration adapted to optimize the formation/cutter contact area while providing a desired volume of formation cutting material.
The use of drill bits for the drilling of wells in earth formations, or for taking cores of formations, is well known. Bits for either purpose may include either stationary cutting elements for cutting or abrading the earth formation, or cutting elements mounted on rotating cones. Bits as presently known to the industry which utilize stationary cutting elements typically use either natural or synthetic diamonds as cutting elements and are known as "diamond bits". References herein to "diamond bits" or "diamond drill bits" refer to all bits, for either drilling or coring, having primarily stationary cutters.
Conventional diamond drill bits include a solid body having a plurality of cutting elements, or "cutters," secured thereto. As the bit is rotated in the formation, the cutters contact and cut the formation. A flow of fluid is maintained through the bit to cool the cutters and to flush formation cuttings away from the cutters and into the annulus surrounding the drill string.
Conventional diamond drill bits may have a variety of different types of cutting surfaces, such as, for example, polycrystalline diamond compact (PDC) cutters, thermally stable diamond product (TSP) cutters, and mosaic-type cutters. Mosaic cutters are typically formed of a plurality of geometrically-shaped thermally stable diamond elements cooperatively arranged and retained in a desired shape, to form a unitary cutter.
With conventional diamond drill bits having such discrete cutters, the cutters are distributed on the bit to provide a desired volume of diamond for cutting the formation. The diamond volume will be determined partially in response to the amount of diamond which will provide adequate cutting of the formation, taking into consideration the wear of the cutters as the formation is cut. Additionally, as is well known, the cutters proximate the outer portion of the bit radius wear much more quickly because of the greater surface velocity as they encounter the formation. Accordingly, outer portions of the bit require much more diamond volume than do inner portions.
Conventional diamond drill bits having discrete cutters include individual cutters distributed across the face of the bit to establish the desired diamond volume. The cutters are distributed in greater numbers along outer portions of the bit radius, to provide greater diamond volume in such areas. Such conventional designs have inherent limitations, however. For example, the volume of diamond, and therefore the number of cutters, required to provide acceptable performance from the bit in terms of wear life, may require an undesirably high weight on bit to cause the bit to penetrate the formation. This is because a large number o cutters providing the diamond volume will also provide a large surface area in contact with the formation which resists penetration of the bit. Additionally, conventional bits, and particularly those with circular cutters, have surface contact areas which increase as the bit wears. For example, when an initial group of five one inch diameter cutters are initially contacting the formation, their curvilinear downward portions will only contact the formation across a chord (contact area), determined by the depth of cut, i.e., the depth to which each of the five cutters actually penetrates the formation. However, when these exemplary five cutters are half worn, their contact area is five full diameters of the cutters. With conventional bits, therefore, as the bit wears, the required weight on bit typically increases, while the rate of penetration typically decreases.
Bits have been proposed for use which have included cutting surfaces with increased depth toward the outer portions of the bit. However, these designs have achieved this increased depth through adjacent squares and rectangles of cutter facing, built up in steps forming large "fins" extending in stair-step blocks away from the body, forming a squared "fishtail" shape. An example of such a prior art bit is found in U.S. Pat. No. 3,059,708 issued Oct. 23, 1962, to Cannon et al. Such proposed designs have not been suitable for the use of different types of cutter facings. Additionally, the design produces a bit having a deep cone stepped profile, in clear contrast to favored generally flat or parabolic bit profiles. Such generally flat bits will be described herein as among those bits having "generally parabolic profiles." Thus, such "generally parabolic profiles," as used herein, may include bits having a generally flat, or slightly downwardly sloping (i.e., shallow-cone shaped) lower surface, as well as bits having upwardly sloping contours, such as, for example, generally "bullet-shaped" bits.
Accordingly, the present invention provides a new drill bit and method for constructing a drill bit wherein the total diamond volume may be varied independently of the diamond volume contacting the earth formation at a given time. Additionally, the diamond volume may be distributed along the radius of the bit to provide an optimal diamond volume at each point along the bit radius.